Gary Fisher crank flex

HI guys,

I just bought a new Gary Fisher Wahoo. I realize the drive train components are only marginal, at best. So far, the bike shifts smoothly and I love the fit and the ride, etc. However, when in the largest front gear ("3") and in 6, 7, 8 of the rear cassette the chain rubs on the side if the front derailleur. This rub only occurs when I apply pressure to the pedals (example, stomping on it going up hill). What is happening is that the cranks (along with the gear unit) seems to be flexing under power. So, while on the bike stand.. no rub occurs I took it back to the LBS where I purchased it and they told me that the crank flex is normal (despite the fact that they couldn't adjust the front derailleur to the point where the rubbing stops. So my questions are.....

1. Is it indeed normal for the crank to flex to the point where chain clearance isn't met in all gear combinations? (I can visually see that the gear unit does not remain parrell to the frame when i put pressure on the pedal (it moves perhaps a few mms ). The "slop" seems to be in the bottom bracket (all nuts, etc are tight). The bike has the stock Shimano MC09 (ie, cheapo) (not sure what kind of bottom braket it is). Would upgrading to a better crank set solve the problem (which seems silly.. since the current one is brand new).

2. The front derailleur is a cheapo (Shimano Acera). Would a better quality front derailleur allow for better clearance/more adjustment? I wouldn't mind replacing the Acera for a better one, regardless..

3. Has anybody else experienced this with their bikes of similar quailty?

My brother has the exact same bike. I adjusted his front derailer H and L stop screws so the chain doesn't rub at either extreme shift position. So far his drive train works great. I don't see any reason to replace it. My old all rigid Fisher's Shimano Deore system shifts crisper and more positive but both get the job done. Adjust your derailer and ride the hell out of it.

Frame Flex....

Unless the tolerances are super close to begin with the crank will not flex enough to cause rub.
What you are running into is frame flex which most people tend to interpret as crank/bb flex. In the gear combinations you mention you are putting tremendous force against the pedals unless you're spun out and going 35mph+. If you look closely you'll notice that the entire BB area is moving side to side including the chainstays and prehaps the spokes in the rear wheel.

The easiest thing to do is not 'stomp on it' Upgrading to a 'stiffer' more expensive crank/bb may help a bit, but it could also make it worse.

Upgrading the f derailleur probably won't have an impact either. If the f derailleur is a bottom swing switching to a top swing may help. This will put the derailleur closer to the BB and it will be more likely to flex the same amount in the same plane as the crank.

I've experienced this on bikes of all quality, from $5k road bikes to $250 mtbs. Its a trait of your frame unfortunately.

Shayne is most likely right. Bro and I are 62 and 66 years old so frame flex isn't an issue with us, lol.
Being a geezer I don't really trust these new fangled aluminum frames. It's all steel and all rigid for me.